1. Field of the Invention
The present invention relates generally to linear induction motors, and, more specifically, the present invention relates to the control of large linear induction motor systems including inductive-based position sensing and stator coil switching algorithms.
2. Description of the Background
Linear induction motors are increasingly being designed for a wide variety of applications from moving loads to launching aircraft. For example, linear induction motors may be used for conveyor systems, material handling, transportation (people movers and trains), projectile accelerators and launchers, and machine tool operations. Large linear induction motors may even be used to launch aircraft from aircraft carriers, and very large “magnetic lifters” are being developed to launch payloads into space. Other applications of linear induction motors include: elevators for aircraft carriers and other locations; conveyors/package handling equipment; torpedo launching systems; and vertical launching systems.
Linear induction motors generally fall into two categories: (1) short primary and (2) short secondary machines. In the case of short primary motors (FIG. 1), the excitation is supplied by a small section of the motor (the primary or stator 105) that moves past a relatively long reaction rail or plate (the secondary or rotor 110). The interaction of the applied currents through windings 115 embedded in slots in the stator 105 and the induced currents in the rotor 110 produce the motor force. As the coils 115 are energized, the stator 105 moves along a track or rail down the length of the rotor 110. Both single-sided (FIG. 1A) and double-sided (FIG. 1B) configurations are possible.
The short secondary motor (FIG. 2), on the other hand, uses a relatively short moving member to act as the reaction rail or plate (secondary or rotor 120) while the excitation is supplied from a relatively long stator 125. The accelerating force acting on the rotor 120 is again due to the interaction of the applied currents in the stator windings 130 and the induced rotor reaction currents. In this case, it is the rotor 120 which is accelerated along a track down the long axis of the stator 125. Again, the short secondary linear motors can be either single-sided (FIG. 2A) or double-sided (FIG. 2B).
In many applications, such as the aircraft launcher, the linear induction motor is characterized by a very long stator or guide rail through which the rotor (also referred to in such orientations as the “runner”) travels down the track. Such a system must be designed to accelerate the aircraft (which can weigh thousands of pounds) to very high speeds in a relatively short distance. As such, the control systems for these large linear induction motors—including stator coil switching and runner position sensing and control—must be designed to maximize performance.
FIG. 3 shows one common method for exciting the windings 130 in the stator 125 in a short secondary linear induction motor. In this approach, current is applied to the entire length of the stator 125 through the embedded stator windings 130. However, this is typically not an effective approach for large linear induction motors because power transfer to the rotor or runner plate 120 only occurs over a small section of the stator 125 (adjacent to the instantaneous position of the runner 120). The rest of the length of the stator 125 acts like a parasitic inductor storing magnetic energy and heating the stator 125 as a result of coil resistive losses.
In addition to the negative impacts on the motor, this approach also requires a larger sizing of the power inverter supplying the electrical energy to the motor. For large accelerator applications such as aircraft or space payload launching (high acceleration of massive objects), exciting the entire stator may require so much electric power that the system becomes infeasible. Consequently, alternative means of exciting the stator local to the region at which the runner exists as it moves along the stator are of interest for this type of motor.
Further, when large linear motors are used, the position of the moving rotor or runner must be accurately sensed and controlled in order to maximize the thrust, conserve power and reduce friction. Even small (˜ millimeters) fluctuations must be compensated. The control system of the linear motor must also be able to compensate for or limit the effect of certain failure conditions, for example when some of the stator windings on one side of a double-sided linear motor fail—causing a magnetic imbalance laterally in the motor.
As such, the present invention, in at least one preferred embodiment, addresses one or more of the above-described and other limitations to prior art linear motor induction systems. The invention preferably includes: systems and methods for appropriately providing current to the stator windings in a long linear induction motor; systems and methods for sensing and controlling the position of a moving rotor in a linear motor; as well as systems and methods for avoiding or limiting certain failure conditions in linear motor systems.